Standard gas burners are known comprising a single flame ring of various dimensions and developable powers.
Special gas burners are also known comprising several concentric flame rings. They are commonly known as wok burners, from the name of the convex-based pans which are widespread in Asiatic countries, and require burners with several concentric rings.
Dual wok burners are also known, provided with special two-way valves for separately feeding the central flame ring and the annular ring or rings with gas, to hence generate either only the inner central flame ring or the central ring and the outer annular ring or rings, in order to vary the cooking intensity within wide limits, from just the inner flame ring fed at minimum level to all the flame rings fed at maximum level for high temperature cooking.
In order for a wok burner to completely cover market needs, it must simultaneously satisfy a large number of requirements, often of a contrasting nature. In particular:                it must be of small vertical height, to be positioned either on cookers or on built-in hobs,        it must be insensitive to the streaming effect, i.e. it must be able to remain always ignited, even under minimum flow conditions, in the case of a reduction in pressure accidentally caused by the effect of opening a door in the kitchen cabinet below the hob,        it must be able to develop high power,        its injectors must be accessible from above for easy replacement if the burner has to be adapted to gas types different from that for which the burner was factory set,        it must be able to operate both as a dual burner, i.e. fed via a two-exit valve, and as a three ring burner, i.e. fed via a normal single-exit valve.        it must comprise just a fed easily assembled parts of low cost,        all its parts must be easily accessible for simple cleaning.        
Various burner types have been proposed to satisfy most of the aforelisted requirements, however they all represent compromise solutions aimed at enhancing certain requirements at the necessary sacrifice of others.
IT 1,232,887 describes a dual burner provided with three injectors associated with three venturis of radial type, i.e. comprising a first vertical frusto-conical section followed by a radial horizontal section. This is of small vertical size, is able to provide reasonable power and uses primary air and secondary air withdrawn from above the hob, hence being substantially insensitive to the streaming effect.
The drawback of this known burner is that the two radial venturis which feed mixture to the outer portion of the burner determine its shape: in this respect, to obtain a radial portion the two venturis are made to extend towards the burner interior, so giving the outer burner portion a shape which is not annular, as is normally required for a wok burner, but presenting a circular outer perimeter and an elliptical inner perimeter. This gives the burner a strange shape and also subtracts secondary air from the central inner part of the burner; consequently the flames at the second ring and at the central burner are very small, hence hardly satisfying the concept of a wok burner, which instead requires considerable flame at the pan centre.
Moreover the three venturis are of small radial extension, with consequent limitation on the maximum power which the burner is able to develop.
U.S. Pat No. 5,401,164 describes a burner of type substantially similar to the preceding and with its venturi too short to develop sufficient power. Moreover this burner withdraws primary air exclusively from below the hob and is hence sensitive to the streaming effect.
U.S. Pat. No. 5,842,849 describes a burner with a cup support closed at its base, on which the injectors associated with the vertical venturis are positioned. The burner withdraws primary air from above the hob, however as the burner is of reduced height in order to be able to be housed within the hobs, its venturis are too short and hence of low efficiency.
EP 1 120 603 describes a burner with a vertical venturi feeding an inner central burner and a diametrical venturi feeding an outer annular burner.
The power developed by this burner is fairly limited; moreover the burner is formed from a very large number of parts and finally, as the two gas inlets feeding the two separate nozzles cannot be connected together, the burner operates only as a dual burner and is unable to function as a single-command burner.
U.S. Pat No. 5,704,778 describes a burner with three horizontal venturis, which is able to develop moderate power, but withdraws the primary air from below the hob and is hence sensitive to the streaming effect.
EP 1 042 634 describes a burner with two injectors feeding a vertical venturi for an inner central flame ring and a horizontal venturi for an outer annular flame ring. The primary air for the vertical venturi is withdrawn from above the hob, whereas the primary air for the horizontal venturi is withdrawn from below the hob.
As the injector holder for the vertical venturi is positioned above the horizontal venturi, the axial length of the vertical venturi is too small and does not enable the burner to develop the required power.
BRIEF SUMMARY OF THE INVENTION
An object of the invention is to overcome all the limitations which are recognizable conjointly or separately in burners pertaining to the state of the art, by providing a burner which satisfies all the aforelisted requirements.